In the past, there has been proposed a gas sensor including a gas measurement chamber, an infrared radiation source, and a detector for detecting infrared radiation. The gas measurement chamber includes an internal wall having a spheroidal shape.
The infrared radiation source is situated at a position of one of focal points inside the gas measurement chamber and the detector is situated at a position of the other focal point (see document 1 [JP 2007-147613 A]).
When a gas to be detected is present in the gas measurement chamber, infrared radiation having a specific wavelength corresponding to a type of the gas is absorbed. Therefore, it is possible to determine the composition of the gas present in the gas measurement chamber based on the magnitude of the infrared radiation detected by the detector.
Besides, document 2 (JP 2006-275980 A) discloses an infrared gas detector including a gas chamber, an infrared light emitting device, and an infrared detection device. The gas chamber has an ellipsoidal shape. The infrared light emitting device is situated at a position of one of focal points inside the gas chamber, and the infrared detection device is situated at a position of the other focal point.
As described above, according to the gas sensor of document 1 and the detector of document 2, light receiving elements are situated at the focal position. Hence, light rays reflected by the surface of such a spheroid can be focused into the light receiving element.
However, with regard to light (infrared light) that is reflected once by part of the surface of the spheroid close to the light receiving element and strikes the light receiving element, an incident angle of such light is larger.
Especially, in the aforementioned detector, an optical filter is situated in front of the light receiving element. The light striking the light receiving element at the larger incident angle also strikes the optical filter at a larger incident angle (i.e., an angle between an incident direction of light on the optical filter and an optical axis of the optical filter). With regard to the light striking the optical filter at the large incident angle, the optical filter is unlikely to show desired transmittance-wavelength characteristics. Consequently, an S/N ratio may be deteriorated. Additionally, an amount of light reflected by a surface of the optical filter is likely to increase.